Potassium thiosulphate

Endpoint summary

Description of key information

Potassium thiosulfate dissociates into thiosulfate anions and potassium cations in environmental solutions.

Potassium is an essential macronutrient in aquatic and terrestrial ecosystems. Dissolved potassium occurs in solution as dissociated K+ ions. Although K is an abundant element, its mobility in soils, sediments and suspended matter is limited since it is readily incorporated into clay lattices and it is adsorbed more strongly than Na+ to the surfaces of clay minerals and organic matter (Salminen, 2005). A European median log Kp value of 3.99 L/kg is derived for sediment-water partitioning of potassium.

Thiosulfate anions are unstable under environmentally relevant conditions and become part of the natural sulfur cycle. Microorganisms control the redox state of sulfur, capable of oxidation or reduction depending on microbial species and environmental conditions. Thiosulfates are readily oxidized to sulfate under aerated conditions and ultimately reduced to sulfide under anoxic conditions (e.g. Lindsay, 1979, Chemical equilibria in soils. Chichester, UK: John Wiley & Sons., Zopfiet al., 2004, Findlay and Kamyshny, 2017, Lee et al. 2007, Barbosa-Jefferson et al. 1998). Regarding the partitioning of sulfur in European soils, data are available from a study by Sheppard et al. (2011) based on data from five different soil types, i.e. clay till, clay gyttia, glacial clay, cultivated peat and wetland peat (n=25), yielding a median logKp(solids-water in soil) of 1.64 L/kg.

Since sulfur exists in streamwater predominantly as the free sulfate anion (Salminen et al. 2005), concentrations of sulfate in streamwater and sulfur in sediment concentrations are applied to examine the respective partitioning. Based on the FOREGS dataset (Salminen et al. 2005), sulfate concentrations of European stream waters are typically below 55 mg S/L (95^thP = 55 mg S/L, 50^thP = 5.6 mg S/L ) whereas sulfur concentrations of sediments are typically below 3,000 mg/kg (95^thP = 2,817 mg S/kg, 50^thP = 508 mg S/kg). The corresponding log sediment/water partition coefficients range from 0.11 to 4.20 with 5th and 95th percentiles of 0.99 and 3.07, respectively (Kd values range from 1.28 L/kg to 15,728.31 L/kg). A European median log Kp(solids-water in sediment) of 2.02 is derived for sulfur.In addition, data is available on marine sulfur partitioning from a reliable, non-guideline study (Sheppard et al., 2011), yielding a sediment-water partition coefficient log Kp(solids-water in sediment) for sulfur of 1.58 L/kg. Results are however based on limited sample size (n=2) and data should therefore be treated with caution.

In soils, sulfur occurs in organic and inorganic forms with the respective ratio depending on soil type and depth. Organic sulfur predominantly occurs in litter in the form of sulfuric acid esters with C–O–SO3 bonds and compounds containing C–S bonds. Sulfuric esters are choline sulfate, phenol sulfates, and polysaccharide sulfates, are of microbial origin and readily available to plants, because they are more easily mineralized compared to C–S compounds that include amino acids such as methionine and cysteine, and sulfolipides. Carbon bound sulfur origins from leaf litter and dead roots and is less mobile and less available to plants and microorganisms because of the strong C-S bond. Depending on moisture and aeration status, different inorganic sulfur forms exist is soil. Elementary sulfur and sulfides are rarely found in well-drained soils because of rapid oxidization to sulfate. In waterlogged soils, reduction to sulfides may take place with subsequent formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, including FeS, ZnS, PbS and CdS (Lindsay, 1979, Finster et al., 1998). Iron sulfides apparently cover soil particles as dark films composed of pyrite (FeS).